The intense electromagnetic field arising at the surface of metallic nanostructures from the excitation of the localized surface plasmon resonance (LSPR) allows for the enhancement of the Raman intensity of adsorbed molecules by a factor up to 4×108 or greater [1]. The structures supporting this plasmonic phenomenon, known as surface-enhanced Raman scattering (SERS), are diverse. The most sensitive examples, with an enhancement factor large enough to observe spectra at the single molecule level [2-5], are aggregated metallic nanoparticles. Correlative structure-activity studies have indeed shown that the presence of a nanometer-sized junction [6, 7] or crevice [1, 8] creates the electromagnetic ‘hot-spot’ (i.e., ‘nanoantenna’) required to observe single molecule SERS. Recent investigations of the hot-spots at the junction of silver cubes [9] and gold pyramidal shells [10] or at the interface between a gold nanostar and a gold surface [11] have highlighted how the control over the structure of this nanometer-scale region is crucial for achieving high enhancement factors. While the early fundamental studies of single molecule SERS have been performed on inhomogeneous samples, the integration of plasmonic nanoantennas into reliable technological applications, such as high sensitivity biological and chemical sensors, requires improved structural reproducibility.
Homogeneous nanostructure populations can be realized via precisely controlled fabrication or post-synthetic sorting techniques. Although much effort has been devoted to the controlled synthesis of nanoparticles, structural polydispersity remains an issue [11-14]. Consequently, post-fabrication separation methods have become important for characterizing or refining populations of nanoparticles based on their size, shape, and aggregation state [15] For example, electrophoretic methods [16], most notably gel electrophoresis [17], have been used to separate metal nanoparticles by both size and shape. Size-exclusion chromatography has also been demonstrated for separating gold nanoparticles by shape [18] and as a tool for characterizing synthesized nanoparticles [19]. In addition, sedimentation coefficient differences between nanoparticles of varying size and shape have been exploited for sorting by centrifugation [20] and sedimentation field-flow fractionation [21]. In particular, a recent study on polymer-coated nanoparticle clusters employed centrifugation and filtration to remove single-core nanoparticles and large aggregates respectively, ultimately yielding samples of primarily multi-core nanoparticle clusters with enhanced SERS signals [22]. Finally, density gradient centrifugation has proven to be particularly successful for obtaining refined populations of nanoparticles, leading to narrow diameter and shape distributions [23, 24] or a specific aggregation state for nanoparticle clusters [25].
For plasmonic applications, the removal of single nanoparticles from aggregates is particularly desirable given that only nanoparticle aggregates (i.e., two or more metallic nanoparticles) have thus far been shown to provide sufficient enhancement for single molecule and single particle SERS [6]. Improved monodispersity within nanoantenna samples allows for increased ensemble SERS signals and removal of inactive species for potential sensing applications. Efforts toward improved monodispersity through controlled synthesis of nanoparticles have been unable to fully address this issue. Previous centrifugal sorting methods for nanoparticles have been limited to slower-sedimenting small diameter nanoparticles and have required chemical functionalization or surfactants to keep the nanoparticles dispersed in solution.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.